Limiting amplifier system



Feb. 14, 1950 E. E. scHRoEDER LIMITING AMPLIFIER SYSTEM 2 Sheets-Sheet 1 Filed Jan. 16, 1948 im omNl l l MHLTUD Jmmwzlou I l l w m l l l l l .l Il.

Inventor-1 Ewald E.Sch1"Oder",

His Attorney.

Patented Feb. 14, 1950 LIMITING AMPLIFIER SYSTEM Ewald E. Schroeder, Berwyn, Ill., assgnor to General Electric Company, a corporation of New York Application January 16, 1948, Serial No. 2,592

2 Claims. (Cl. 179-171) My invention relates to a klimiting amplifiery system and has particular utility in a peak limiting audio amplifier.

In many applications, particularly in the audio amplier circuits of broadcast transmitters or of sound recording equipment, it is important to provide linear amplification up to a predetermined maximum amplitude level and a high degree of volume compression without loss in fidelity for all signals exceeding this level. It is a primary object of my invention to provide an improved amplifying system of this type which closely realizes these objectives and which prevents audible thumps or click-s from appearing in the output signals in response to sudden input signal peaks exceeding the desired level.

It is also an object of my invention to provide an improved limiting amplier system which is very fast-acting, which provides a high compression in modulation level above a desired limiting threshold level, and which is characterized by very low waveform distortion both under transient and steady-state conditions.l

In prior art limiting ampliiiers, difficulty has generally been encountered on certain types f program material due to the phenomenon known as pumping In order to prevent a noticeable reduction in gain in an audio signal below the limiting level which immediately follows such a high-intensity peak or signal of high level, the system must have a fairly fast gain recovery time. If the input signal contains a series 0f closelyspaced high-intensity peaks there will therefore tend to be produced a relatively rapid shifting of gain, or pumping This is especially noticeable when a series of words are spoken against an applause background. rI'he up and down effect of the gain reduction upon the relatively steady applause background is then most annoying. It is therefore desirable to provide a system having a gain recovery characteristic which is automatically varied with the type of program material. Specifically, the recovery time should be automatically increased in the presence of repeated overmodulation peaks in order to reduce the variation and gain between peaks. It is therefore another object of my invention to provide an improved peak limiting audio amplier in which the gain recovery time is made an automatic function of the characteristics of the program signal so as to reduce pumping effects.

For additional objects and advantages, and for a better understandingof the invention, attention is now directed to the following description and accompanying drawings, and also to the appended claims in which the features of the invention believed to be novel are particularly pointed out.

In the drawings: Fig; 1 is'a schematic circuit diagram of a limit- ""25 and 26.

ing amplifier system embodying the invention; l

and

Fig. 2 is a graph illustrating certain gain limiting characteristics of the circuit of Fig. l.

Referring now particularly to Fig. 1, the major elements of the limiting amplier system are represented within dashed rectangles. audio signals are supplied through transformer 9 from any suitable audio source, not shown, to

a preamplifier I0. Amplified signals are coupled through transformer II to the input of a signal delay network I2. The delayed signals are then supplied through an attenuator network I3 and a transformer I4 to a controlled amplier I5, in which the gain limiting action occurs in a manner shortly to be described. The output of amplifier I5 is then coupled through a transformer I6 to the pair of output terminals I1 which are connected to any suitable load circuit, not shown. For example, the load circuit may comprise the modulator of a broadcast transmitter or a sound recording apparatus. The output of pre-amplifier III is also coupled directly over conductor I8 to a control bias generator I9. Generator I9 'contains circuits for developing gain limiting control potentials which are applied to control amplifier I5 over conductor 2U. Suitable operating potentials for the pre-amplifier I0 and control ampliiier I5 are supplied from a unidirectional "source, not shown, between a positive bus B+ tential with respect to the common ground bus f G. The power supplies should preferably be wellregulated.

Having thus generally described the major 'components of the limiting amplifier system, the

detailed circuit arrangement will now be considered. Pre-amplier I0 may be any suitable audio amplier known to the art. As represented, it comprises two resistance-coupled stages The amplitude of the input signals supplied to stage 26 may be adjusted by moving tap 23 on potentiometer 24, which is connected across the secondary of transformer 9. In accordance with preferred practice, a certain Ikamount of degenerative feedback is supplied from the output of stage 26 to the input of stage 25 through network 21 in order to improve the stability of the amplifier and the delity of reproduction. The purpose of the pre-ampliiier I0 is to raise the level of the input signal, which may be supplied from a very low-level source such as microphone or sound recording head, to a level suiiicient to supply both the controlled amplifier I5' and the control bias generator I9.

The delay network I2 may also be of any suit--v Input 'y able conventional design, and is represented as a lattice network. The audio signal voltage is delayed byythe networkfl2 andytheinputleyel, to the controlled amplier I5 may.,.be. independ-v ently adjusted by means of a variable element of attenuator I3, represented conventionally by.y

arrow 28.

The controlled amplier I5 is a.- two-stage,- push-pull amplifier comprising a, pair op elecg.. tron discharge devices 3| andA 32invthe input.

stage and a pair of electron discharge devices 33 and 34 in the output stage, The two stages` are resistance-coupled in conventionalmannerand it is believed that the circuit connectionsg and the manner of applying operating potentials Will .baobvioosito,thosefskillodin the art With.-

outl detailed explanation. A certain amount of degeneration is provided by utilizing unbypassed cathode resistors 35 36 and 31, 44,l 43 in the re-4 s pectivel stages.

Amplifier.. I5 ,is given a.va1iable gairrchar-v acterisvtic by. utilizing awvariable amountrof degenerativeV voltage. feedback between the 'two' stages.. The.l feedback pathsv comprise a pair of y .es itsanQde connected tothe anode yof one of the.Y output vdevices 33, 34 and its cathode connected triodel'discharge devicesand 39 eachhaving tQthe cathode yof a corresponding one of thede-V vices.3|, 32H.: Thecontrol, grids of the devices 3B Y,

and`39 are connected.- to ground through portionsvof. potentiometers 40 and 4I, respectively, and` common resistor` 42. Control bias potentials` developed inlcontrolbias generator I9 in a ,man-f nor. toabo.- des;ololloed-.Ioo.1o.\i.,v oro .impressed .upon

3.51 The.. onodo-to-.oethodo impedonoes..

potentiometers l4l!. .a1fid 4 l in ,paralleL overcondooior.. 20. of ,.devices.38,.,39.are,thereby controlled. This in turn .varieslthe amount. of l feedback 'from the.. outputtoi the.inputstage of amplifier I5 and.

thus.. its.over.all gain.. The connectionfrom potenriometer u teeomrorgridcf. device 'asis proierobly modoy adjustable, as. i1lusiratod...for.

balancing purposes.:

prefamplier .l I il Dare supplied over L conductor I 8 to nooiod .to BT. through; resistors 52. .and 53- Reare supplied throughcoupling capacitors 54 and 55 y.to theicontrolgrids of lagpiai'rof. dischargedef and, SI1, 6I..respectively. Thedevices aretriode-l connected and :the signal output `ris taken ,from

theirv cathodes. andi supplied throughy blockingY 55 vices@ and 51... The .anodesof devices 55. and.l 51. areconnectedto ground and-thecathodes arev connected-.to B,- through load. resistors '58,r 58,.

20, discharge device 93... having. a.Y left-handjsection.

capacitors 5,2and553to a full-,wave rectier comgprising. a pair of diodes 54, 65.-Y Thusd'the Edevices iossouroeiorih .diodes 641.455..i

tetes .ol-ao@ in. fdiories n. /i, 65.- anci a looiiirol bias.

potentiel-Lis developed. aorossiho. ...diode v.load net- ..A

Wok'. 10..,- A....Drima1:v .Component of the network...

'IQ ,.is.. afdiode load. capacitor 'I I, connected from .270. the anooesoi.diodos.oiaiidto.Baand .S1.1111,1.i.ei... briioioiois J2., 1s and ,liir-ioriosf. Capaoiio l5 oriotlc ier-.Shoot io..11'osisiors.12. 13 and. .ro-l sistor 14 respectiyelg, n are v small audio bypass A positive delay bias is impressed upon the cathode of diodes 64, through resistors 19 and mrespectively; This D Qtcntialgisgsupplied from point BI `on',' 1,vo ltti9re divider network comprising resistors 82 and 83 connected across the power supply buses. This bias potential determines the threshold limiting level of the amplifier for any given input signal level, since diodes 64, 65 cannotc.ondu ct until the-applied audio signals exceed this level.

For reason s -.that will shortly appear, the diode load network 'IIIK also includes a resistor 90 and capacitor 9-I serially connected across the diode loadresistors l2, 13, 14. This series circuit connection forms an antifpump circuit, as Will become. shortlyapparent.

A portion of the contro1bias-yoltase, appearing.. across resistor 14, is impressed over conductor'- 9 2;on. the left-,handcontrol gridofduo-triode 93d anda lrightehand .section 93h. Another por tionof` this control bias potential, appearingy across.. resistors/13, and` 14, is similarly supplied, overa conductor, 94 to theright-hand control;v ,grid of section 9327. Section 93a and its assos.

ciated vcircuits operate as a` `polarityinverter in order. to obtain a control voltage .ofthe correctI polarity for application tovthecontrol grids of.

the feedbackcontrol devices 38and39.A It .will

be. observed that A the anode of section. 93a. is con: nectedto ground through ,the parallel resistors,

Miland v4I andE common lresistor 42. The, cathode is vconnectedto vB-.- through anetwork ccmprising resistor4 94'r inseries with an adjustable A resistor95 yand fa parallel resistor. 95, theseref sisters allnbeingshuntedby a, capacitorf91. As? will be 4discussed more fully later,v ,this networkv not only provides an operating grid bias for. section .93a but lalso `performs an. imp OItant function in .suitablyrshapngthevoltage pulses ap.

"plied to. die. controlled amplifier. I 5.

AThe right-handsection 93h of dischargede-- vice. 93is cormected1 in a vcircuit t0 indicate `the degree of xgain-,reduction .upona calibrated VU meter IDI).y The.anodeofvsectionbis connected toground through. resi stor Inl xand the cathode.. is Vconnected, tdB- throughr potentiometer |02.. Theme ter I Udmeasures `a portion of. the current inl resistor 4I 02,.

Under these condi-7 generator`1 I9 .draws la certain .normal value of.

1 anode current, and 4thereby provides .a.. certain "grid bias voltage forieedback control devices 38 1. and 39. "Ihis in turnresults in a predetermined; gain,througlgr controlledy amplier -I 5, .which may g beconsideredthenormal gain ofthe system;

Assume novi/.that a single,sharpsisnalpeak cc.- ourswhioli overoomesthe threshold limiting level'.

The funi-wave .f rectiiiergli, ,65. now draws current through its,=lowfmpedance ,drivingrsource and Charges. .oapaotorl l This .charging l action .may ff be `made extremely fast, if the drlvng impedance is kept.. low through. the. use4 of. cathode-.coupledoutputiitouitsior .tubes 5E. and .51.A andthe. use... of power-type amplifiers for these tubes. A Inother words the time constant of the charging Lnetwork fqr. capacito11.1 I.. isrfmadefvoryfast1.f of;..theorder y Thevoperationand adjustmentpf. the system., maybest be understood by nrstconsideringthat signals,v within, the 4audio frequency band, e., g from, about 3Q to 15,0v00fcycles per second, are impressed,iioouinputtransformor .I 0 and that these .signals do not eXedhe ldelaybias level in the..`

control bias .generator I9. tionsrectiersllsand 65.are non-conductive and section 93a. 0f the output stage in. control bias `be termed the slope control. load network. comprising resistors 49, 4I and 42,

i5 of microseconds) so that a sudden overmodulation peak causes a control voltage pulse of relatively steep wavefront to be impressed upon the control grid of amplifier section 93a. The polarity of this control voltage pulse is such as to render the control grid of section 93a more negative with respect to ground, thereby decreasing the anode current and increasing the potentials impressed on the control grids of devices 38 and 39. Consequently, the amount of negative feedback between the two stages of the control amplifier I is increased, reducing its gain.

The rate of change of anode current in section 93a, in the presence of such a control bias pulse of steep wavefront, is also determinedby the time constant of network 94, 95, 95, 91. The dynamic characteristic may be adjusted over a predetermined range by adjusting resistor 95, which may Since the anode is substantially non-reactive, it has very little effect on the wave shape.

Capacitor 91 further provides a low cathode impedance for section 93a at the time of initial application of bias voltage, and therefore increased gain for section 93a during the rise time of the pulse. The bias voltage applied to devices 38 and 39 is therefore initiallyin'creased in the presence of transients of steep wavefront. in-

creasing the speed of the control action considerably.

By proper adjustment of the circuit constants and operating voltages, the gain-reducing action in the controlled amplifier I5 may be made inherently as fast, or faster than, that of control ance value for resistor 90, of the order of several megohms. The sum of resistors 12, 13 and 14 is also proportioned with respect to capacitor 1I so as to provide a discharge circuit for capacitor 1I of desired time constant at the end of the pulse. It is desirable to make this time constant relatively short so as to restore the original amplifier gain fairly rapidly after a modulation peak, though not so short as to cause waveform distortion. The time constant of this network upon discharge may for example be of the order of .2 to .4 second. This corresponds to the period of a very low frequency below the lower limit of the audio frequency band but is still sufficiently fast to :avoid depressing the gain -of the system for an appreciable period following the modulation peak.

Now, if several high signal peaks of short duration occur during a short interval, the capacitor 9| as well as capacitor 1I will become charged, causing gain reduction as before. Following this series of peaks, the initial rate of gain recovery will be nearly as fast as the recovery following a single peak up to approximately 40% of the total gain recovery. This follows from the logarithmic discharge characteristics of the networks, providing an initial high rate of discharge in either case. When the ratio of the potential on capacitor 1| to that on capacitor 9| is equal to the ratio of the sum of resistors 12, 13, 14 to the sum of resistors 12, 13, 14, 90, the remaining recovery time will be determined by the discharge characteristics of the anti-pump circuit. As previously mentioned, the discharge time constant of this network is preferably of the order of several seconds', for example 6-10 seconds. It can thus be* seen that when a series of short signal peaks occur in rapid succession, the anti-pump circuit will be effective to limit the amount of gain recovery between peaks, thereby reducing the pumping effeet. A similar effect will be produced in the event of a sustained signal peak of high amplitude which causes both the capacitors 1I and 9| to be charged. The advantage of the lengthened recovery time for these conditions is that less dynamic range will be lost and the effects due to signal compression will be less apparent to the listener than might otherwise be expected.

Fig. 2 illustrates the volume compression characteristics of the amplifier of Fig. l, based upon experimental data. Relative output signal level in decibels is plotted as ordinates against relative input signal level in decibels as abscissa. The characteristics for an amplifier having constant gain are represented by the dashed line |06. The gain reduction in decibels at any particular signal input level is then the vertical distance D between this dashed line `and the solid line curve |01 representing the actual limiting amplifier characteristics. It -will be seen that the system embodying my invention provides a very high order of compression ratio within the operating range. As is evident from Fig. 2, the output level is held constant within .5 db. for an input level increase of l2 db. above the threshold value.

The time delay of the audio signals in passing through network I2 is made just long enough so that the fast-acting control bias generator reduces the gain of control amplier I5 before the input signals are impressed upon amplifier I5. In this way, no voltages exceeding a predetermined value can appear at the amplier output under any conditions, even on very high modulation transients of steep wavefront.

The threshold limiting level may readily be changed by adjusting tap 23 on potentiometer 24. This varies the signal input level of both controlled amplifier I5 and control bias generator I9. The gain of amplifier I5 may then be separately adjusted to secure any desired gain characteristic within the operating range by readjusting variable attenuator element 28.

Merely by way of illustration, the following are some of the more significant circuit constants which have been found to be suitable in the control bias generator I9:

Tube 5I): Type 6J5 Tubes 5E, 51: Type 6Y6 Tubes 64, 65: Type 6X5 Tube 93: Type 6SN7 Capacitors 62, 63:2 mfd. Capacitor 15::.001 mfd. Capacitor 16:.0039 mfd. Capacitor 9I=.1 mid. Capacitor 1I=.02 mid. Capacitor 91:.05 mid. Resistors 58, GI=1000 ohms Resistors 59, 60:3500 ohms Resistor 12:3.3 megohms Resistor 13:4.'7 megohms Resistor 14:2.0 megohms Resistors 19, :2(),000 ohms Resistor 82=48,000 ohms Resistor 83:1,600 ohms Resistor 94:4,700 ohms Resistor 95:3,300 ohms (variable) Resistor 96:3,900 ohms Another important advantage of my improved limiting amplifier system is the low waveform 1,?distortiorit1-both undertransien'tandfsteadyestate conditions; it is'I'Wellflknown4 that` the Jdistortion f'ff an uampliiler` vis idecreasewwith i1 an: increased amount of :negativefeedback."l.'Sincel` the effect -of over-modulationis'i'toincreaselthernegative V5 :causesaideimpedance.tozdecrease. ieedb'ack;the-signaldistortion infamplier I5 is rmactually: reduced# as :.ltheiasignal kinput level .vincreases abovexthe; threshold limiting value.

`'While a certain" specific; embodimentl has been :Letters A'Patent of thUnited States :is:

-. '1. Aalimitingfaniplier system'comprising;l in

combinationlan lLaudio ifrequencyasignal input rcircuit; afsignalf, delay. network; L a wmulti-stage 'i 20 .said.:.'diode, fa'iirst resi'storinshunt totsaid capaciaudi'olamplier?havingfaanf. input coupled to said circuit throughv said network, a degenerative feedaback .1path--between two; stages' of' saidv amplifier, saidf-path including the; anode-toecathodeimpe'dsanceofa,fgrdcontrolledfdischargetd'evce, a ccn- 25 across"saidlrst'resistor and'forming-.therewitna :trol biasgeneratorthavingan input supplied difre'ctlyf romssaidl -inputf circuit-and" comprising4 a dow-impedance v.signal rectifying :device and `a ca- =:pacitance`resistance load i network', Ameans for medal-:cents sadgridcontrolled .zischargeffdevice -in-t responseatoi saidbiasi'yoltagegvsaid delaynetwork "providing :a: time;Y delayY lgreaterl than .the time required fon.saidfcontrolxbias. generator" to 2. Alvp'eak` limiting;"amplifyingV sytemfy compris- "ing, in combination, a'isource ofzaudiowfrequency signals, .az-'signal 'i' delay network, a; multistage 4audiof amplifier coupled: toi said f source ii through -sh'ownfl and describedfitvwfll.ofilcoursebezunder- 10 l'saidunetworkza 'degenerative feedbackr network efstood: that'rvariousx:modifications may be: made 1withoutdeparting 'from the invention. .Thezapvpended claims are iutheret'ore` intendedV to .'cover vany-such modifications'withinthe true 4spirit-,and ffsoope'l .ofi the? invention.

i between two 1 stages fof said r ampliera including the anode-to-cathode path of aA variable; impedi. ance f devicez having at' control electrode, ai- .series :,p'eak tidetector :having zan'finput circuit' supplied 15 vwith;:undelayedf:audio signals fromr said source sand' comprisinga .diode and la capacitor in series, said input circuitf having aV very shorttime Vconstant 'relative t@ tlregperiod' ofan audioi frequency fsigna1f.du1ing.l 'charging ofr said capacitor? through torzand forming; therewith anoutputA circuit having a discharge time constantof the' orderlofsO fsecond, anA antii-:pumpcircuit comprisingzzasecond resistorsanda second capacitor'connected inseries rnetwork havingv arelatively "longtimei constantA f the order' of"several.secondsfand means for mpressing'unidirectionali.controrpotentials"responsive tot" the potential acrosssaid output circuit biasing'saidnrectifyingl device tornon-conductivefgu on' said'.control'fele'ctrode.

r condition :unti'lirappliedzzaudio frequency Vsignals ffexceed'faf predetermineddevel; said loa-d' network w being-1charged'tlirough; saidf rectifying .device vto frrprovide'fa biasfvoltagein' response tolaudio signals exceeding saidl'eVeI; said :network having a short 35 time'constanton' chargefandia' long time constant on :discharge relativei t'oxth'e period of-a 'low audio frequency 'isignal, fan; antipump network comprising a series: resistor andi capacitor connected ,1, 925 608 in shunt" to'saidiload network aridfhaving a time *40 constant substantially longerithan said discharge timer constant: y1anclemeans` tfor reducing the irn- EWALD E. SCHROEDER.

"REFERENCES CITED ""Th'e followingreferences are of recor'din the 'le'of this patent:

^ UNITED STATES L".EATENTS Number j Name nDate .fRound Sept. 5, 1933 21,244,695 Hathaway June 10,1941 A .L 2,283,241 Van Cott. V '.\May 19,'. 1942 2,343,207 Schrader et al.: I Febi` 29, 1944 

